A shuttle device utilizing a long span lead screw and nut drive has been used to transmit heavy torque and thrust loads at relatively high speeds. One such device is shown in U.S. Pat. No. 3,670,583 in the name of John C. Lemming, which includes a non-rotatable coactive nut operatively mounted on a rotary screw and adapted to travel freely linearly along the screw. The problem with conventional lead screw and nut drive assemblies is that the nut threads have to be machined to match the threads of the lead screw. At high speeds, the nut drive and lead screw may vibrate due to the tolerance between the lead screw and nut drive threads. Also, misalignment of the nut drive and lead screw may occur due to the machine intolerances of the threads of the nut drive. Further, large amounts of power are required to drive various loads due to the short lead in a single start.
U.S. Pat. No. 4,790,971 to Brown et al teaches a method of casting nut threads, and a corresponding apparatus, to minimize such problems. The method involves forming polymer threads in the cavity of a nut complementary to the threads on a section of the lead screw. The threads are formed by centering a section of the screw in a nut having a cavity with a diameter larger than the diameter of the screw threads, by filling the cavity with a liquid polymer and by allowing the polymer to dry and cure around the threads of the lead screw.
Unfortunately, when the polymer is introduced into the nut cavity around the threads of the lead screw section, air pockets and air bubbles form as a part of the threaded polymer insert. The presence of air pockets and air bubbles in the polymer insert affects not only the life of the threaded polymer insert but also the mechanical strength of the polymer insert.